[bootloader] Improve description of *efiBootloaderId* option

src/modules/bootloader/bootloader.conf

@@ -20,8 +20,16 @@ timeout: "10"
 grubInstall: "grub-install"
 grubMkconfig: "grub-mkconfig"
 grubCfg: "/boot/grub/grub.cfg"
-# Optionally set the --bootloader-id to use for EFI. If not set, this defaults
-# to the bootloaderEntryName from branding.desc with problematic characters
-# replaced. If an efiBootloaderId is specified here, it is taken to already be a
-# valid directory name, so no such postprocessing is done in this case.
+
+# Optionally set the bootloader ID to use for EFI. This is passed to
+# grub-install --bootloader-id.
+#
+# If not set here, the value from bootloaderEntryName from branding.desc
+# is used, with problematic characters (space and slash) replaced.
+#
+# The ID is also used as a directory name within the EFI environment,
+# and the bootloader is copied from /boot/efi/EFI/<dirname>/ . When
+# setting the option here, take care to use only valid directory
+# names since no sanitizing is done.
+#
 # efiBootloaderId: "dirname"